1. Field of the Invention
This invention relates generally to an optical sensor for flame detection, and more particularly, to a high temperature silicon carbide based sensor for use in gas turbine engines for detecting the presence of a combustion flame and/or an afterburner flame.
2. Brief Description of Related Prior Art
The most common method for flame detection in gas turbine engines, either for aircraft and marine propulsion or for land based power generation, is by a light activated or photosensitive gas-tube discharge detector. Such detector consists of a phototube having a cathode which is photoemissive (i.e. it emits electrons when illuminated), and an anode for collecting the electrons emitted by the cathode. The tube is filled with a gas at low pressure which is ionized by any accelerated electrons. A large voltage potential is applied to and maintained between the cathode and the anode such that in the presence of a flame, photons of a given energy level illuminate the cathode and cause electrons to be released and accelerated by the electric field, thereby ionizing the gas and causing amplification until a much larger photocurrent measured in electrons is produced. If the electrical field is continually maintained, the device will conduct electrons even when the photons stop, until the metal surfaces are depleted, thus functioning in a manner similar to the "start circuit" of a gas discharge lamp. To ensure tube longevity and to prevent faulty "on" indications, a large AC voltage is applied through a resistor/inductor circuit to the sensor. In this manner, the device acts like a single wave rectifier since the tube only conducts when both the voltage is high and photons are present. When the voltage drops, the tube stops conducting and will not conduct any further photocurrent until the next time it is illuminated in the presence of photons. Although such an apparatus is well proven, having been used reliably for many years in a wide range of applications, specifically with gas turbine engines, it has several drawbacks.
Typically, a very large AC potential in excess of 200 volts is required. This equates to a power drain of approximately 7 watts, by far the largest energy user of all engine sensors. Moreover, such an apparatus suffers from degraded performance when the surrounding temperatures exceed 400 degrees Fahrenheit due to changes in the properties of the metal components at such elevated temperatures. Accordingly, some gas turbine engines require the use of specifically dedicated cooling equipment to maintain the sensor temperature at acceptable levels. Such collateral cooling systems, however, place additional demands on the system, and add to its cost, weight, and complexity, and decrease its reliability.
Another system for flame detection, specifically for detecting the presence of an afterburner flame in augmented gas turbine engines, is disclosed in U.S. Pat. No. 4,510,794 to Couch. The Couch system relies on an ion/electrostatic probe which provides ionic flame detection and electrostatic engine wear monitoring by measuring the conductivity through the plasma of the afterburner flame.
As modern electronic systems have replaced archaic tube-based hardware with semiconductor components, photodiodes have been implemented in applications for measuring or detecting the presence of light throughout the visible and ultraviolet spectrum. Their smaller size, greater stability, enhanced reliability and lower cost make them vastly superior to the phototube.
The photodiode is a p-n junction device with an associated depletion region wherein an electric field separates photogenerated electron-hole pairs, the movement of which generates measurable current. When actinic electromagnetic radiation strikes the semiconductor material, the electron-hole pairs are generated. When these charge carriers are generated near a p-n junction, the electric field of the depletion layer at the junction separates the electrons from the holes in the normal p-n junction fashion. This separation produces a short circuit or open circuit voltage, technically referred to as the photovoltaic effect. Such photodiodes are of the type disclosed in U.S. Pat. No. 5,093,576 to Edmond et al.
U.S. Pat. Nos. 5,303,684 and 5,257,496, both to Brown et al, disclose a combustion control system for controlling the level of N.sub.ox emissions produced in the combustion process to reduce such emissions while maintaining a sufficiently high combustion flame temperature. This is achieved by monitoring the intensity of ultraviolet spectral lines associated with the combustion flame, and then dynamically adjusting the fuel/air ratio of the fuel mixture. These patents describe in a general sense the use of silicone carbide (SiC) photodiodes to measure light intensity in a system for generating a signal correlating to the NO.sub.x emission concentration for adjustment of engine operating parameters. However, these patents do not disclose specific sensor structures and methods for fabricating same that are reliable in and for use in the high temperature environment near the combustion section or afterburner section of a gas turbine engine, wherein the engine surface temperature may reach 550 degrees Celsius for extended periods of time, in which environment the sensors are intended to operate.
Accordingly, it is an object of a present invention to provide an optical flame sensor comprised of a silicon carbide detector and silicon carbide amplification circuitry in a unitary assembly for attachment to the wall of a gas turbine engine.
It is yet another object of the present invention to provide an optical flame sensor designed to operate for extended periods of time in a high temperature environment without the need for additional cooling equipment.
It is still another object of the present invention to provide an optical flame sensor of compact and light weight construction and enhanced reliability.
It is yet another object of the present invention to provide an optical flame sensor for detecting the presence of afterburner light-off in afterburning gas turbine engines, wherein the sensor contains optical filtering means for rejecting electromagnetic radiation in excess of 280 nanometers to eliminate false indications caused by incident solar radiation.
It is a further object of the present invention to provide an optical flame sensor disposed within the center-body fairing of a fuel/air premixer in an aeroderivative gas turbine engine.
It is another object of the present invention to provide a method for fabricating an optical flame sensor that is especially adapted to permit the sensor to operate for prolonged periods in high temperature environments without degradation in performance of the sensor.